Oscillator circuit



May 14, 1935. P. o. FARNHAM OSCILLATOR CIRCUIT Filed Sept. 23, 1931 May 14, 1935. P. o. FARNHAM 2901594 oscILLAToR cIRcUIT Filed Sept. 23, 1931 2 Shee'cs-SheefI 2 /r'/a Cyc /es Carl-[er Frequency W (/MLM,

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Patented May 14, 1935 UNITED STATES VPA'r-Elxi'r oFFlcE' oscILLA'roR omoUrr Paul O. Farnham, BoontonN. VJ'., assignor, Vby

mesne assignments, to Radio 'Corporation 'of AmericayNew York, N. Y., a corporation of Delaware Application September 28, 198.1, serial Ne. 564,676

9 olaims. (cl. 250-20) and local oscillator circuits which exhibit a sub-v stantially constant frequency difference when tuned overl a band of frequencies by similar sec- 10 tions of a gang condenser. The usual methodfor securing tracking of the circuits is to' employ a relatively complex oscillator circuit having an inductance somewhat less than that of the carrier circuit, and having-a large capacity in series with and a small capacity shunted across a tuning condenser which is substantially identical with the carrier frequency tuning condenser. In actual practice, the large series capacity usually takes the form of a large fixed condenser shunted by a small adjustable condenser, since the lseries capacity will be of the order of 1000 micromicro- 'farads vin the case of receivers for signals in the 550 to 1500 kilocycle band, and must be adjustable to effect alinement of the oscillator andcarrier frequency circuits.

An object of the invention is to provide an oscillator circuit adapted for use in a single control superheterodyne receiver and which is of simpler and less expensive construction than the known oscillator circuits. Another object is to provide a tuned oscillator which will deliver 'higher output voltages as the oscillator is tuned to lower frequencies. More specifically, an object is to provide an oscillator circuit having coupled 35'grid' and plate coils which are wound and connected in the same sense, the phase reversal between grid and plate voltages necessary for oscillation being obtained by making the reactance of the leakage inductance of the plate coil larger 40 than the, capacity reactance between plate and cathode for all frequencies in the tuning range of the oscillator. A further specific object is to provide a superheterodyne receiver in which the oscillator voltage impressed on therst detector increases automatically with decreasing frequency, thereby offsettingothe frequency Variation of signal energy transmitted to the first detector. These and other objects of the invenion will be apparent from the following specification vwhen taken with the accompanying drawings, in which: Fig. 1 is a fragmentary Vcircuit diagram of a superheterodyne receiver embodying the invention,

Fig. 2 is an equivalent diagram of the oscillator v circuit, and

Fig. 3 is a curve sheet showing the Variation, with frequency, of theoscillator voltage and the translation gain of the first detector. w

vIn 'the drawings, the reference numeral identies a vacuum tube for amplifying signal energy at carrier frequency, the amplified energy being passed to the detector tube 2 and there combined with local oscillations generated by oscillator tube 3 to produce a beat or intermediate frequency. With the exception of the oscillator circuits, this portion of the superheterodyne receiver may be of any appropriate design. In the .and preferably is, vsubstantially identical with the amplifier input circuit, the adjustable capacities beingi usually provided by Sections of a gang condenser, Vas is indicated by the broken line 4 which connects the tuning condensers of the amplifier and detector stages. The beat frequency set up in the detector is passed, as by coupling transformer 5 to the intermediate frequency amplifier tube 6.

The oscillator circuit includes a tuning conq denser C shunted across the grid inductance L1 'i which is coupled tothe plate inductance L2. As indicated by broken line l, the adjustable element of condenser Cis mechanically connected to 'thetuning condensers C of the amplfier and detector stages and the several tuning condensers are preferably similar sections of a gang condenser. o

The oscillator tuning condenser is shunted by a small condenser C0 which, as in' the described prior arrangements, is initially adjusted to secure the desired frequency difference between the oscillator andV carrier frequencies when the tuning condensers C are adjusted to tune the circuits to their respective higher frequencies. Theplate grid and plate supply leads are connected to 7tor circuit in any appropriate manner. shown, `a coil L3` is coupled to the oscillator grid ground by condensers 8 of the order of 0.1 microfarad.- i

The cathode circuit includes a bias resistor 9 of about 7,500 ohms, shunted by a condenser IO of about 0.001 microfarad. VThe tube 3 is prefer- Vably a screen grid tube, since substantial capac-VV ity coupling between the plate andcontrol grid will, if not neutralized,cause the circuit to stop oscillating at the higher frequencies.,

`It is particularly -to be noted that the grid and plate coils L1, Lz are wound and connected Vin the same sense, which arrangement is contrary to the usual oscillator practice. A phase reversal of grid and plate voltages; withzrespect to the f cathodevoltage, which is necessary for oscilla-1 tion,' is obtained by the action of the leakage inductance ofthe plate coil L2 and the plate-' cathode capacity` C. This will be apparent from the circuitidiagram. Fig. 2, which shows,

a. net :WorkV` equivalent to that of the oszuvlator circuit. The ,direction of the osciilatory current flow is indicated'by the arrows and, for

i current flow as indicated, the polarities at points in the circuits are indicated by the plus and minus signs, the cathode potential Vbeingrtakien as zero.

In one particular circuit' for use covering the band of from .550 to 1500 kilecycles, L1 had a value of 139 microhenries and L2 was 453 microhenries, the Vcoupling producing a mutual inductance M of about rnicrohenries. The tuning condensers C were sections of a 500 micromicrofaradgang icondenser, and the condenser Cz was adjustable from to 220 micromicrofarads and consisted of a small trim- Vming condenserof the type used in adjusting intermediate frequency transformers.

The oscillatory voltage may be introduced into the first detector stage by coupling to'the deteccoil and included in the detector cathode-circuit between the cathode and the bias resistor whichis shunted by condenser l2. 2

InV general, the oscillator grid inductance L1 isysomewhat smaller than the inductances L of the tunedcarrier. circuits, an appropriate ratioj of Li to L being about 0.83.' As stated above, the trimming condenser C0 of the oscillator is adjusted to obtain the desired tracking of the oscillator and carrier 'circuits' at the high frequency end of the band, and the plate cathode 'capacity Cz is adjusted for trackingat the low frequency end. It is to be noted that capacity Cz is'comparatively small and is provided by a singleV Ucondenser, which isin Contrast to the prior' superheterodyne oscillator circuits employing a high series capacity which, to obtain the desired preliminary adjustment, usually consists of a fixed and'va'riable condenser;V

The oscillator'lias the special property 'of gen- Lerati'ng avvoltage which is a function of the frequency, varying approximately inversely with the frequency. In Fig. 3, the curve A represents the Variation, with Vcarrier frequency,`of the osi cillatory voltage introduced into the cathode cirf cuit of the first detector.A VAssuming that the carrierrrvoltage input to the first detector remains constant over thefre'quency band, curve" Bshcws the variation,twith carrier frequency, of

the translation gain in theiirst detector stage.

It will be noted -that the translation gain is approximately vproportional lto the oscillator voltage, and increases with decreasing frequency.

The significance'of this is that the antenna input in i a receiverV plate' voltag'es'.

i whereby p 2,oo1,894

system may be designed for maximum eiiiciency as regards energy transfer and noise suppression.

- Such input systems exhibit a marked frequency Variation of transmission and it has been customary to employ less efiicient and/or more expensive circuit arrangements to secure a gain at the lower frequencies which was not substantially less than the gain at the high frequency end of the Vband. Thepresent invention avoids this vnecessity for specialinput systemsby automatihaving a connectionto the cathode,an adjustable condenser shunted across the grid coil; and means comprising a condenser connected between-plate and cathodefor effectingia reversal of the phase of the alternating voltages developed at the grid andv plate terminals of the coupled coils.

2.A Vvacuum tube system comprising an oscillator-having at least one grid and a plate in' addition to a cathode, inductively coupled grid and plate coils wound in the'same 'directionrand similarly coupled to grid and plate, an adjustable condenser shunted across the grid coil, and ca.-

pacity between plate and cathode resonating withtheleakage inductance of the'plate coil to effect a reversal` ofthe phase of the grid and 3. A vacuum tube system comprising an oscillator having at least one grid and a plate in addition to a cathode, inductively coupledcoilseach having one terminal connected to the cathode by circuit elements which maintain the sameat the potential of the cathode for the' oscillation frequency, the two other terminals of said coils being connected respectively to said grid and plate, the sense ofv the inductive coupling between said coils being such as to give the said other termiinals the same open-circuit polarity, and' capacity connected across'each'of'the respective coils and 'of such magnitude as to tune each coil individually to substantially different frequencies,

the phase of the voltage between the grid and cathode is made'opposite to .that between the plate and cathode. i

4. In an oscillatorsystem, claimed in claim 3, wherein one of said capacities is adjustable toV vary the frequency of oscillation. i q i q 5. In an` oscillator 'systerm'the invention as claimedin claim 3, wherein the Vcapacity connected across one coil is of fixed value, and the other capacity is adjustable'tov vary the: frequency of oscillation over a frequency range higher than the natural frequency to which said fixed value I capacity tunes its associated coil.v

6. In a radio transmission system, the combination with a tuned circuit includingan induci' tance and an adjustable condenser, of a vacuum tube having a plate electrode and at least one grid lelectrode cooperating with a cathode, a

tuned `impedance network .associatedV with said tube; said network comprising a pair of magnetically coupled inductances connected between the cathode and the respective electrodes, a tuning condenser connected across one of said coupled the invention as' inductances, said tuning condenser being substantially identical with said adjustable condenser and mechanically connected therewith for simultaneous adjustment of the said condensers, and fixed condensers shunted across each of said coupled inductances to establishes a frequency interval between the resonant frequencies of said tuned circuit and tuned network as said condensers are simultaneously adjusted.

7. In a superheterodyne receiver, the combination with a radio frequency circuit including an inductance shunted by one section of a gang tuning condenser, of an oscillator system adapted to resonate at a frequency spaced by a fixed in- -terval from the resonant frequency of said radio frequency circuit as saidrtuning condenser is adjusted, said oscillator system comprising a vacuum tube having a plate electrode and at least one grid electrode cooperating with a cathode, magnetically coupled inductances connected between the cathode and the respective electrodes, a second section of said gang tuning condenser connected across one of said inductances, and a relatively fixed capacity connected across the other inductance.

8. In a superheterodyne receiver, the combination with a radio frequency Circuit including an inductance shunted by one section of a gang tuning condenser, of an oscillator system adapted to 'resonate at a frequency spaced by a fixed interval from the resonant frequency of said radio frequency circuit as said tuning condenser is adjusted, said oscillator system comprising a Vacuum tube having a plate electrode and at least one grid electrode cooperating with a cathode, magnetically coupled inductances connected between the cathode and the respective electrodes, a pair of inductively coupled coils wound in the same direction, the two adjacent terminals at one end of said coils being maintained at the oscillating potential of the cathode, the other terminals of said coils being connected to the respective plate' and grid electrodes, circuit elements shunting a second section of said gang tuning condenser across 'the grid coil, and means comprising a fixed capacity connected between the platev electrode and cathode for effecting a reversal of the phase of the alternating voltages developed at the plate and grid terminals of the coupled coils. o

9. In a superheterodyne receiver, the combination With a radio frequency circuit including an inductance shunted by one section of a gang tuning condenser, of an oscillator system adapted to resonate at a frequency spaced by a fixed inter- Val from the resonant frequency of said radio frequency circuit as said tuning condenser is adjusted, said oscillator system comprisingV a vacuum tube having a plate electrode and at least one grid electrode cooperating with a cathode, a pair of coils each having one end maintained at the alternating potential of the cathode, the opposite ends of said coils being connected to the plate and grid electrodes respectively, coupling between said coils of such sense as to give the plate and grid terminals of said coils the same open-Circuit polarity, circuit elements shunting a second section of said gang tuning condenser across the grid coil,'means reversing the phase of the oscillating voltages developed at the plate and grid terminals of said coils, a'nd means comprising a fixed capacity connected between the plate electrodeand cathcde for maintaining the oscillator frequency spaced from the resonant frequency of said radio frequency circuit by a fixed interval as said gang tuning condenser is adjusted.

' PAUL O. FARNHAM. 

